1. Field of the Invention
The present invention relates to a liquid developer set which is used for developing an electrostatic latent image with a liquid developer in an electrophotographic image forming device such as a copying machine or a printer, and is formed of the liquid developer as well as a concentrated liquid developer and a diluent to be supplied to the liquid developer. Further, the invention relates to the concentrated liquid developer to be supplied or added, a diluent to be supplied or added, and a method of manufacturing the diluent.
2. Description of the Background Art
For electrophotographic image formation, an electrostatic latent image is generally formed, e.g., by effecting image exposure on an electrostatic latent image carrier such as a photosensitive member in accordance with image information, and then is developed into a visible toner image, which is transferred and fixed onto a record member for providing an intended image.
The developing method in the electrophotographic image formation can be classified into a dry developing type and a wet developing type.
In the dry developing method, the developer is formed of toner or a mixture of toner and carrier having, e.g., magnetism. Usually, the dry toner includes, as its major components, a pigment and binder resin, and may also include a charge director, a conductivity control agent, a plasticizer and/or a releasing agent and others which are internally or externally added thereto, if necessary. Further, the magnetic toner includes magnetic powder such as magnetite (Fe.sub.3 O.sub.4). In the dry developing method, the toner is usually charged by contact with a specific surface of a developing device and/or mutual contact between toner particles. In the case of two-component developer including carrier, the toner is usually charged by contact with the carrier or the like. Further, charging may be performed, e.g., by electrostatic induction by an electric field, implantation of charges, and/or ion absorption caused by ionizing and discharging of air or the like. The toner thus charged is transferred onto an electrostatic latent image carrying portion of an electrostatic latent image carrier such as a photosensitive member by an electrostatic force, a mechanical force, a magnetic force or the like, and is used for development by an electrostatic force.
Since dry toner and others used for the dry development may escape into and float in an ambient atmosphere, diameters of them can be reduced only to a restricted extent, and is usually in a range from about 5 .mu.m to about 10 .mu.m. Since the dry toner has a relatively large diameter, a high image density can be easily achieved.
Meanwhile, the developer, which is currently mainstream in the wet developing method, includes an electrically insulating dispersant medium (carrier liquid), and also includes the toner for development (typically, coloring fine particles primarily formed of a pigment and binder resin), a charge director, a dispersion stabilizing agent and others which are dispersed in the dispersant medium. It is considered that the toner in the wet developing system is charged owing to ion absorption by virtue of the charge director, and the charged toner electrically migrates in the carrier liquid owing to the electric field applied to the liquid developer, and is transported onto the electrostatic latent image carrier for use in development.
Since there is no possibility that the toner used in the wet development escapes into the atmosphere, fine toner can be used, and it usually has an average diameter of about 5 .mu.m or less. Thereby, the produced image can have a high resolution and a good gray scale property. Also, advantages such as easy fixing of the toner image can be achieved.
The toner in the liquid developer exists as particles, of which surfaces are charged to have a certain polarity by virtue of the charge director. It can be considered that ions of the charge director or ions produced by reaction of the charge director with the toner are present as counter ions having the polarity opposite to that of the toner. Accordingly, the toner is usually charged to a higher extent as the amount of the charge director added to the carrier liquid increases. By applying a voltage to the liquid developer, the charged toner and the counter ions in the carrier liquid electrically migrate in the opposite directions, respectively. The development is performed by utilizing this phenomenon (electrophoresis).
According to the study by the inventors, it is already known that all the charge director added to the liquid developer is not absorbed to and reacted with the toner to contribute charging of the toner. A large part of the added charge director is present in the carrier liquid. This can be confirmed by the following experiment. Hydrocarbon solvent of a high resistance of about 10.sup.14 .OMEGA..multidot.cm is used as the carrier liquid, and the liquid developer prepared by adding the toner and the charge director to this solvent is processed to separate the toner and the liquid component from each other by centrifugal separation. When an electric resistance of the liquid component thus separated is measured, it usually exhibits a value smaller than 10.sup.14 .OMEGA..multidot.cm.
With increase in amount of the added charge director, therefore, the amount of the charge director absorbed to and reacted with the toner increases, and the amount of the free charge director existing in the carrier liquid also increases so that the electric resistance of the carrier liquid lowers. As already described, development of the electrostatic latent image on the electrostatic latent image carrier is developed with the liquid developer in such a manner that the charged toner electrically migrates in the carrier liquid owing to the electric field applied to the liquid developer, and electrostatically adheres onto the electrostatic latent image carrying portion. In this operation, if a large amount of free charge director is present in the carrier liquid, the free charge director acts to erase the electrostatic latent image so that image flow (blurring) occurs and/or image density lowers.
In the image forming apparatus using the liquid developer, a predetermined space is formed between the electrostatic latent image carrier carrying the electrostatic latent image and a developing electrode for development, and the liquid developer is supplied into this space. When the voltage is applied across the electrostatic latent image carrier and the developing electrode, the charged toner electrically migrates in the carrier liquid, and is absorbed onto the electrostatic latent image carrier, and free charge director is left in the carrier liquid. Accordingly, the free charge director is condensed in the liquid developer when the development is repeated.
Since the toner in the liquid developer is consumed, the toner must be supplied or added. For supply of the toner, the device uses a concentrated liquid developer (which may also be referred to as a "concentrated liquid" hereinafter) having a toner concentration which is about 10 times larger than that of the liquid developer used for usual development, and thereby a constant toner concentration is kept in the liquid developer used for the development.
For manufacturing the liquid developer in the prior art, such a method is employed in view of the manufacturing cost and the easy handling that the concentrated liquid is first prepared, and then is diluted with the carrier liquid. Accordingly, the concentrated liquid contains the charge director in higher concentration than the predetermined concentration of the charge director in the liquid developer used for the development.
For performing the image formation while supplying the concentrated liquid to the liquid developer, the concentrated liquid is first diluted with an organic solvent formed of the carrier liquid, and the diluted liquid is used as an initial developer or starter. When the image density lowers, the concentrated liquid is supplied. When the amount of the liquid component of the liquid dwveloper decreases, the carrier liquid is supplied. According to this method, the toner concentration can be kept constant, but the charge director is accumulated in the liquid developer used for the development. When only the concentration of the charge director increases while the toner concentration is constant, an amount of charges of the toner usually increases excessively, or an electrical resistance of the carrier liquid usually lowers excessively. In either case, the density of the final image extremely lowers when the image formation is repeated many times.
As a supply method which can suppress the above phenomenon, U.S. Pat. No. 4,860,924 and others have disclosed a method, in which the concentration of the charge director in the concentrated liquid is lower than a conventional value, and an independent carrier liquid solution of the charge director is used for supplying the charge director. In this method, a predetermined voltage is applied to the liquid developer used for the development, and a current flowing therethrough is detected. Based on the detected current value, the concentrated liquid and the carrier liquid solution of the charge director are supplied independently of each other so as to keep the current at a constant value. According to U.S. Pat. No. 4,860,924 and others, the current kept at the constant value flows through the liquid developer so that change in image density can be suppressed.
However, even if the value of the current flowing through the liquid developer is kept constant according to the method taught by U.S. Pat. No. 4,860,924 and others, the image density gradually changes in a practical operation when the concentrated liquid and the carrier liquid solution of the charge director are supplied.